Material loss monitor for corrosive environments

ABSTRACT

The present invention relates to monitoring the corrosive damage caused to items in a corrosive environment by using multiple monitored coupons  30  of varying thickness that, when corroded, transmit information to a monitor  40  allowing a user to estimate the corrosive damage valuable industrial components  20 . The corrosion rate of the coupons  30  can be used to optimize the environmental conditions, so that items in the environment corrode at desired rates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This invention claims priority to U.S. Provisional application60/528,875, filed on Dec. 11, 2003, entitled “Material Loss Monitor,”which is incorporated herein by reference.

FIELD OF THE INVENTION

The field of the invention relates to monitoring the damage caused toitems in a corrosive environment. More particularly, the presentinvention estimates damage to parts in a corrosive environment and aidsin determining optimal environment conditions.

BACKGROUND

Many industries have machinery and parts that are intermittently orcontinuously exposed to corrosive environments. Due to the nature ofthese environments, direct observation of the corroding parts is usuallyimpossible since they tend to be in inaccessible areas. In most cases,actual measurements of corrosion levels require that entire systems bebrought off line and opened up.

Bringing systems off line, however, can be very expensive and timeconsuming, therefore corrosion rates tend to be estimated. Since failureof some parts can be catastrophic, the corrosive levels of the parts areoverestimated which results in the operation of the processes in whichthey are used are kept on the conservative side. This reduces the riskof a failure due to excessive corrosion, however by overestimating thecorrosion levels, process efficiency is typically lost. Therefore,without a more accurate estimation of corrosion levels, efficiency needsto be sacrificed to prevent part failure.

An example of a such system that has a corrosive environment is shown inFIG. 1. This figure illustrates a heat recovery steam generator (HRSG),which is used to turn otherwise wasted hot gasses into useful steam. Hotgasses enter 2 the HRSG from sources such as a gas turbine (not shown).Depending on the nature of the fuel used in the gas turbine, the hotgasses will contain varying levels of corrosive substances. In a HRSG,the hot gasses pass over heat transfer surfaces made up of tubes inwhich water, under pressure is converted to steam. The steam rises inthe tubes and is collected in a series of three drums, a high pressure(HP) drum 6, an intermediate pressure (IP) drum 8, and a low pressure(LP) drum 10. Ultimately, the hot gasses are vented 4 after beingdepleted of most of their useful heat.

In the type of HRSG represented in FIG. 1, the heat from the hot gassesare transferred to either water, steam, or a combination of water andsteam through use a boiler tube, which is a type of heat exchanger. Anexample of a typical boiler tube 20 at the rear of the HRSG is shown inFIG. 2. In this figure, the hot gasses 24 first pass over evaporationtubes 12 that heat water contained in the tubes to the point where thewater is converted partially into steam. This water-steam mixture entersthe LP drum 10, where the steam is separated 11. In this process, the LPdrum requires make up water 22 to replace the volume lost by conversionto steam 11 and other causes. In order to capture the greatest amount ofheat from the now cooled exhaust gas, the coldest water entering thesystem is first heated by passing it through a boiler tube. The boilertube 20 runs water 22 through a center passage, while the hot gasses 24pass over the outer surfaces, usually comprised of fins 26, to transferthe heat from the gas to the water. The hot gasses 24, however, containcorrosive elements, such as sulfur, that will erode the boiler tube 20.If the boiler tube corrodes to the point of breach, the entire HRSGsystem becomes subject to damage and will need to be taken offline.

The boiler tubes that carrying water to the LP drum 10 of the HRSG areparticularly susceptible to corrosion, since at lower temperatures,generally between 110–250° F. (43–121° C.), corrosive elements condenseout of the hot gas and form acids. In fact the section of the boilertube that first receives the make up water 28 is the most susceptible tocorrosion because it is the coldest part of the boiler exposed to thehot gasses 24. The HRSG system has a life expectancy of about 25 years,and if a boiler tube fails before this time, replacement costs anenormous amount of time and money. To prevent failure of the boiler tube20, the temperatures of the tubes carrying water to the LP drum 10 arekept higher than they may other wise be, so that less corrosive elementscondense out of the hot gas, causing a higher temperature gas to bevented to the environment, which is a waste of heat energy as well as asource of heat pollution. Over the course of 25 years this represents asubstantial loss of revenue to the operator, and the waste of energy andexcess heat pollution can have a large negative effect on theenvironment.

What is needed is a method and apparatus that can monitor the corrosionlevels of component parts in corrosive environments.

SUMMARY OF THE INVENTION

With the foregoing in mind, methods and apparatuses consistent with thepresent invention, which inter alia facilitates the monitoring of thecorrosion level of part in a corrosive environment. Machinery parts in acorrosive environment are notoriously difficult to observe, since thecreation of the corrosive environment generally requires separation fromthe standard environment. The difficulty in observing corrosion can leadto whole systems failing due to corrosion of essential part. To overcomethis, in the prior art corrosion level had been estimated. The estimatesof the prior art, however, tend to be on the conservative side andresult in inefficiencies either by running systems at sub-optimallevels, making more expensive parts or by replacing corroding parts toosoon.

These and other objects, features, and advantages in accordance with thepresent invention are provided particular embodiments by a plurality ofcoupons of varying thickness that are exposed to the same corrosiveenvironment as the part that is desired to be monitored. Oftentemperature is a factor in determining corrosion rate. The corrosiveenvironment of the coupon is kept the same temperature of the part bythe intimate contact of the coupon with the part so that their corrosionrates are the same. As the coupons are corroded through, they trip amonitoring device, which will notify a user of the coupon's failure.Since the coupons are of varying thickness, the user may chart the rateof corrosion and estimate when failure due to corrosion will occur. Insystems such as a HRSG where corrosive conditions directly correlatewith system efficiency, a maximum corrosive environment can bemaintained without a premature failure of the monitored part.

In one embodiment the present invention provides for a method ofmonitoring corrosion of a part in a corrosive environment. The methodcomprises installing on the part multiple coupons of varying thickness,where the coupons are at least in part exposed to the corrosiveenvironment. The coupons are linked to a monitoring device, which may beremote from the corrosive environment, where the monitoring device willregister a failure in at least one of the multiple coupons. The failureis then interpreted as an approximate level of corrosion in the part.

In one embodiment the present invention provides an apparatus formonitoring corrosion of a part in a corrosive environment. The apparatuscomprises one or more coupons that are exposed to the corrosiveenvironment in a manner similar to the part. The apparatus furthercomprises a monitoring device that registers a failure in the one ormore coupons. The coupons are either mounted to the part, the monitoringdevice, another feature in the environment or a combination thereof. Thefailure of a coupon correlates to an approximate level of corrosion inthe part.

In a particular embodiment the present invention provides for a methodand apparatus of monitoring corrosion of a boiler tube in a heatrecovery steam generator. The method comprises installing on the boilertube multiple coupons of varying thickness, where the coupons are atleast in part wrapped around a surface of the boiler tube that isexposed to the corrosive environment. The coupons maintain substantialthermal contact with the boiler tube and are linked to a monitoringdevice. The monitoring device will register a failure in at least one ofthe coupons, where the failure in at least one of the coupons is abreaking of at least one of the coupons. The registering of the failureof at least one coupon comprises placing a coupon under tension thatholds a spring under tension at a terminal end of the coupon, where thebreaking of the coupon releases the spring, and the release of thespring closes a particular circuit, and where the closure of theparticular circuit indicates a specific failure of a coupon of specificthickness. The specific failure in the coupon of specific thickness isthen interpreted as an approximate level of corrosion in the boilertube. In a more particular embodiment multiple interpretations are madeon multiple failures of coupons of specific thickness to determine anapproximate rate of corrosion in the boiler tube.

BRIEF DESCRIPTION OF THE FIGURES

The invention is explained in more detail by way of example withreference to the following drawings:

FIG. 1 illustrates a heat recovery steam generator according to theprior art.

FIG. 2 illustrates a typical boiler tube used in a heat recovery steamgenerator according to the prior art.

FIG. 3 illustrates a material loss monitor according to one embodimentof the present invention.

FIG. 4 illustrates one embodiment of a material loss monitor mounted toa boiler tube.

FIG. 5 illustrates one embodiment of a device for determining thefailure of a coupon.

FIG. 6 illustrates an embodiment of a material loss monitor where thedevice for determining the failure of a coupon is remote from thecorrosive environment.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides for a corrosion rate monitoring methodand apparatus. In one embodiment the invention provides multiple couponsof varying thickness that are exposed to the same corrosive environmentas a part that is desired to be monitored. As the coupons are corrodedthrough, they trip a monitoring device, which will notify a user of thecoupon's failure. Since the coupons are of varying thickness, the usermay chart the rate of corrosion and estimate when failure due tocorrosion will occur. In systems such as a HRSG where corrosiveconditions directly correlate with system efficiency, a maximumcorrosive environment can be maintained without a premature failure ofthe monitored part.

Referring to FIG. 3, one embodiment of the present invention is shown inwhich the material loss monitor comprises multiple coupons 30 that drawan elliptical path and connect to a monitoring device 40. In thisembodiment, the entire material loss monitor can be placed into thecorrosive environment. A second monitoring device (not shown) outside ofthe environment and connected to the first monitoring device 40 allows auser to check the status of the coupons 30.

In FIG. 3, the coupons 30 are arranged to wrap around a cylindrical partsuch as a pipe or boiler tube. In this embodiment, one end of the coupon30 is attached to the monitoring device 40 via an eyebolt 34, while theother end is attached back to a fixed point on the monitoring device 40using a dowel 32 which provides tension to the coupon 30. In thisembodiment the eyebolt 34 may be part of the monitoring device 40 and ina in a particular embodiment is intricate to determining coupon failure(discussed below). In this manner, a plurality of coupons 30 areattached to an eyebolt 34 wrapped partially around a part to bemonitored, and then pulled taut using the dowel 32 and attached back tothe monitoring device 40. The free end of the coupon could also beattached to another anchor point, however other anchor points may not bereadily available when the material loss monitor is being installed.

Also shown in FIG. 3 is a mounting device 36. In this embodiment themounting device is installed on the part to be monitored. Thisembodiment of the device being mounted is also illustrated in FIG. 4,where the mounting device 36 is viced to a boiler tube 20. In otherembodiments, the mounting device can be a variety of objects anddesigns, such as a clamp, vice, chain, stand or integrated with the unitto be monitored such as a tab In particular embodiments, the mountingdevice aids in keeping the coupons 34 taut as well as stabilizing themonitoring device 40.

In the FIGS. 3–4 the coupons 34 appear to be uniform wires, which is forillustration purposes only, and in many embodiment the coupons will beof varying thicknesses and a variety of shapes. In certain embodiment,when it is desired to monitor a particular corrosion point, it ispreferable to have a single coupon of a certain thickness, or multiplecoupons of a similar thickness. In alternate embodiments, however, it isdesirable to use the present invention to monitor successive stages ofcorrosion, and multiple coupons of varying thickness are used. Theinitial coupon thickness is a known measurement, and is linked to aparticular point on the monitoring device. In particular embodiment, thecoupons are thicker in places along their length where failure isundesired. For example, in FIG. 4, it is desirable to measure thecorrosion where the coupon 34 touches the boiler tube 20 where thecorrosion rate is the greatest. However, in certain environs theportions of the coupons are entirely exposed to the open corrosiveenvironment, they may fail faster than the part of the coupon that is inintimate contact with the monitored part. Therefore, in this embodimentthe thickness of the coupons not in contact with the monitored part isincreased to prevent premature failure at that point. Alternately, thecoupons can be insulated along the length where premature failure isundesired.

Coupon shape is also varied depending on the embodiment. In manyembodiments a wire coupon may be too fine and fail mechanically fromvibration or other mechanical forces and may also corrode too quicklydue to the large surface area per volume. Ribbon-shaped coupons aretherefore preferred in some embodiments as they add to the cross-sectionarea without adding thickness to the coupon. In addition theribbon-shape coupon provides a more precise measurement of corrosion inrelation to thickness since the corrosion at the sides of theribbon-shaped coupon have little effect on the coupon's failure. Othershapes of coupons include, but are not limited to spiral, square,chain-link or twisted strands.

To further increase the accuracy of the measurement of corrosion inrelation to coupon thickness, coupons are pulled taut across themonitored part. This limits the exposure to one side of the coupon tothe corrosive environment. Also, this allows for thermal conductivitybetween the coupon and the monitored device. As discussed in FIG. 2, aboiler tube 20 conducts cold water 22 through its center, and is exposedto hot gasses 24 on its exterior. It is the cold nature of the boilertube that allows for corrosive elements to condense onto its surface. Insuch circumstances, coupons need to be tight against the boiler tube sothat they stay at essentially the same temperature. It is important,however, not to over-stress the coupons, since many materials corrodefaster when they are stressed. The material loss monitor may beinstalled in place for years or decades, so even a small corrosiveincrease can have a long term effect.

Coupons may be in contact with monitored parts in a variety of differentway. They can be wrapped several times around a part to be monitored, orthey can touch it on an arc. In FIG. 4 the coupons 34 are attachedaround the circumference of the boiler tube 20, though in otherembodiments they may run partially or completely down the tube's length.In still other embodiments the coupons are placed in the corrosiveenvironment without contact with any specific part. Often parts havecontoured surface areas such as fins. The coupons of the presentinvention can go between the contours and contact the tube where thecorrosion is the greatest.

The material of which the coupons are made can be varied depending onthe nature of the corrosive environment and the composition of the partto be monitored. Some examples include, but are not limited to, carbonsteel, metals and alloys, synthetic fibers and plastics. In particularembodiments, the coupons are made from the same material as the partsthat they are monitoring.

As discussed above, some parts in corrosive environments have lifeexpectancies of years or decades. If parts fail early then entiresystems can be disrupted. However, if the corrosion levels are estimatedtoo cautiously then efficiency is wasted. By using coupons of varyingthickness, the present invention can be used to optimize operation ofsystems in corrosive environments. For example, in a HRSG, coupons maybe arrayed such that they are estimate to break every six month or everyyear. If coupons are breaking earlier than expected, then thetemperature of the gasses surrounding the boiler tube or the temperatureof the fluid within the tube can be increased. If the coupons arelasting longer than expected then more of heat from the hot gasses canbe recycled.

Referring to FIG. 5, one embodiment of the monitoring device thatdetects coupon failure is shown. An eyebolt 34 anchors one end of acoupon (not shown). The tautness of the coupon compresses a tensionspring 44. While the coupon remains intact, the tension spring 44remains compressed. However when the coupon fails, the tension spring 44releases and closes a circuit (not shown). The tension spring isprotected from the corrosive environment by a membrane 42 that preventscorrosive elements from entering the monitoring device. The termtensioned spring as used herein alternately refers to compressedsprings.

The monitoring device detects a coupon failure when a circuit is switchfrom an “off” state to an “on” state. In other embodiments, the failureof a coupon will switch a circuit from an “on” state to an “off” state.In particular embodiments, the coupon itself comprises part of thecircuit, so when the coupon is broken the circuit is opened. Extracaution needs to be taken for this embodiment, however, since running anelectrical current through a coupon can itself increase corrosion, andthe electrical current may fail before the coupon is actually broken.

The monitoring device itself may have its own self monitoring systems.For example, an active circuit within the monitoring device that issusceptible to corrosion will fail if corrosive elements enter themonitoring device. This type of self monitoring circuit can be placed inproximity to vulnerable areas, such as the membrane 42 that protects thetension spring 44 from the corrosive environment. This can beparticularly useful in embodiments where a monitoring circuit needs tobe turned “on” rather than “off” as described above.

The expression coupon failure as used in the above examples generallyrefers to the breakage of a coupon from corrosion. However, additionaltypes of failure may also indicate corrosion depending on theembodiment. Such as a slacking of the coupon or a failure to carry asufficient current.

Referring to FIG. 6, one embodiment of a monitoring device 46 that isremotely placed from the corrosive environment is illustrated. Multiplecoupons 30 partially wrap the circumference of a boiler tube 20 that isexposed to a corrosive environment. The coupons then leave the corrosiveenvironment through a wall 50 and attach to a remote monitoring device46. The monitoring device 46 may display the status of the coupons 48 onits face, or transmit the status to a more remote location (not shown).In this embodiment, the ends of the coupons may be physically checkedand tightened when necessary.

In one embodiment the present invention provides for a method ofmonitoring corrosion of a part in a corrosive environment. The methodcomprises installing on the part multiple coupons of varying thickness,where the coupons are at least in part exposed to the corrosiveenvironment. The coupons are linked to a monitoring device, which may beremote from the corrosive environment, where the monitoring device willregister a failure in at least one of the multiple coupons. The failureis then interpreted as an approximate level of corrosion in the part.

In one embodiment the coupons are ribbon-shaped. In a relatedembodiment, the varying thickness of the coupons is from 0.1″ (cm) tothe failure thickness of the part. In another embodiment the coupons arein thermal contact with the part, and in a related embodiment thecoupons are at least partially wound around the part.

In another embodiment, the monitoring device registers the failure by atleast one of the coupons breaking. The breaking of at least one of thecoupons releases a tensioned spring, and the release of the tensionedspring closes a circuit. The closing of the circuit then is theregistering of the failure. In a particular embodiment each of themultiple coupons has a specific circuit that registers a specific couponbreak. In still another related embodiment an electrical current ispassed through the coupons, and the monitoring device registers thefailure by the coupons breaking, where the breaking of at least one ofthe coupons interrupts the electrical current. In some instances thecorroded coupons may fail to carry the electrical current beforebreaking, which will also be registered as a failure.

In a particular embodiment the present invention provides for a methodand apparatus of monitoring corrosion of a boiler tube in a heatrecovery steam generator. The method comprises installing on the boilertube multiple coupons of varying thickness, where the coupons are atleast in part wrapped around a surface of the boiler tube that isexposed to the corrosive environment. The coupons maintain substantialthermal contact with the boiler tube and are linked to a monitoringdevice. The monitoring device will register a failure in at least one ofthe coupons, where the failure in at least one of the coupons is abreaking of at least one of the coupons. The registering of the failureof at least one coupon comprises placing a coupon under tension thatholds a spring under tension at a terminal end of the coupon, where thebreaking of the coupon releases the spring, and the release of thespring closes a particular circuit, and where the closure of theparticular circuit indicates a specific failure of a coupon of specificthickness. The specific failure in the coupon of specific thickness isthen interpreted as an approximate level of corrosion in the boilertube. In a more particular embodiment multiple interpretations are madeon multiple failures of coupons of specific thickness to determine anapproximate level of corrosion in the boiler tube.

In one embodiment the present invention provides an apparatus formonitoring corrosion of a part in a corrosive environment. The apparatuscomprises one or more coupons that are exposed to the corrosiveenvironment in a manner similar to the part. The apparatus furthercomprises a monitoring device that registers a failure in the one ormore coupons. The coupons are either mounted to the part, the monitoringdevice, another feature in the environment or a combination thereof. Thefailure of a coupon correlates to an approximate level of corrosion inthe part.

In a related embodiment, the apparatus comprises multiple coupons, andthe monitoring device is capable of registering a separate failure ineach of the coupons. The coupons can either be free standing from thepart being monitored or they can be in intimate contact with the part.

In another related embodiment, the coupons are of a variety of thicknessor a variety of materials, or both. In particular embodiments, thecoupons are of the same or similar material as the part being monitored.The coupons can be all attached to the same monitoring device, ormultiple monitoring devices can be used. In some embodiment the samecoupon is attached to more than one monitoring device.

Though particular embodiments of the invention discussed have focused onHRSG systems and their corresponding boiler tubes, the present inventionis equally applicable in other corrosive and erosive environments. Theterm corrosive, as used herein, includes erosive, caustic, wearing andsimilar environments. A variety of parts may also be monitored, such aspipes, containers and walls, erosion.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. Accordingly, theparticular arrangements disclosed are meant to be illustrative only andnot limiting as to the scope of the inventions which, is to be given thefull breadth of the claims appended and any and all equivalents thereof.

1. A method for monitoring corrosion of a part in a corrosiveenvironment comprising: installing on said part a plurality of couponsof varying thickness, wherein said coupons are at least in part exposedto said corrosive environment, wherein said coupons are at leastpartially in thermal contact with said part, and wherein said couponsare at least partially wound around said part; linking said coupons to amonitoring device, wherein said monitoring device will register afailure in at least one of said plurality of coupons; and interpretingsaid failure in at least one of said plurality of coupons as anapproximate level of corrosion in said part.
 2. The method of claim 1,wherein said plurality of coupons are ribbon-shaped.
 3. The method ofclaim 1, wherein said failure of at least one of said plurality ofcoupons consists of a breaking of at least one of said plurality ofcoupons.
 4. The method of claim 1, wherein the varying thickness of saidcoupons is from 0.1″ (0.25 cm) to the failure thickness of said part. 5.The method of claim 4, wherein each of said plurality of coupons has aspecific circuit that registers a specific coupon break.
 6. The methodof claim 5, wherein an electrical current is passed through saidplurality of coupons, and wherein said monitoring device registers saidfailure by at least one of said coupons breaking, wherein the breakingof at least one of said coupons interrupts said electrical current.
 7. Amethod for monitoring corrosion of a part in a corrosive environmentcomprising: installing on said part a plurality of coupons of varyingthickness, wherein said coupons are at least in part exposed to saidcorrosive environment; linking said coupons to a monitoring device,wherein said monitoring device will register a failure in at least oneof said plurality of coupons; and interpreting said failure in at leastone of said plurality of coupons as an approximate level of corrosion insaid part; wherein said monitoring device registers said failure by atleast one of said coupons breaking, wherein the breaking of at least oneof said coupons releases a tensioned spring, and wherein the release ofsaid tensioned spring closes a circuit, wherein the closing of saidcircuit is the registering of said failure.
 8. The method of claim 1,wherein said coupons are made of a similar material to said part.
 9. Themethod of claim 1, wherein said monitoring device is remote from saidcorrosive environment.
 10. A method for monitoring corrosion of a boilertube in a heat recovery steam generator comprising: installing on saidboiler tube a plurality of coupons of varying thickness, wherein saidcoupons are at least in part wrapped around a surface of said boilertube that is exposed to a corrosive environment within said heatrecovery steam generator, wherein said coupons maintain substantialthermal contact with said boiler tube; linking said coupons to amonitoring device, wherein said monitoring device will register afailure in at least one of said plurality of coupons, wherein saidfailure in at least one of said plurality of coupons is a breaking of atleast one of said plurality of coupons; wherein the registering of saidfailure of at least one coupon comprises placing a coupon under tensionthat holds a spring under tension at a terminal end of said coupon,wherein said breaking of said coupon releases said spring, and therelease of said spring closes a particular circuit, wherein the closureof said particular circuit indicates a specific failure of a coupon ofspecific thickness; and interpreting said specific failure in saidcoupon of specific thickness as an approximate level of corrosion insaid boiler tube.
 11. The method of claim 10, wherein a plurality ofinterpretations are made on a plurality of failures of coupons ofspecific thickness to determine an approximate level of corrosion insaid boiler tube.
 12. An apparatus for monitoring corrosion of a part ina corrosive environment comprising: at least one coupon; a mount forsaid coupon; and a monitoring device operatively coupled to said couponsuch that it is capable of registering a failure in said coupon; whereinsaid coupon is exposed to said corrosive environment in a manner similarto said part, wherein said coupons are at least partially in thermalcontact with said part, and wherein said coupons are at least partiallywound around said part; wherein said failure of said coupon correlatesto an approximate level of corrosion in said part.
 13. The apparatus ofclaim 12, wherein said apparatus comprises a plurality of coupons,wherein said monitoring device is capable of registering a separatefailure in each of said plurality of coupons and wherein said separatefailure of said plurality of coupons correlates to an approximate levelof corrosion in said part.
 14. The apparatus of claim 13, wherein saidplurality of coupons are of a variety of materials.
 15. The apparatus ofclaim 13, wherein said apparatus further comprises a plurality ofmonitoring devices.
 16. The apparatus of claim 12, wherein said couponis made of a similar material to said part.
 17. The apparatus of claim12, wherein said coupon is in intimate contact with said part.